1. Field of Invention
The present invention relates to a method for making barium titanate powder. More particularly, the invention provides a method of making barium titanate powder by thermally decomposing a barium titanyl oxalate compound formed by adding barium acetate to a solution of oxalic acid and titanium oxychloride.
2. Description of Related Art
Barium titanate (BaTiO3, hereinafter sometimes abbreviated as “BT”) is one of the most widely used materials in electronic and microelectronic applications due to its excellent ferroelectric, piezoelectric, and dielectric properties. For example, BT powder is often used in the fabrication of multi-layer ceramic capacitors (hereinafter sometimes abbreviated as “MLCC's”), thermistors, and other electroceramic devices.
The trend toward continued miniaturization of electronic components has placed increasing demands on the BT powder used to fabricate such components. For example, as the fired dielectric thickness of MLCC's continues to decrease (e.g., to less than about 3 microns), it becomes necessary to make a finer BT powder with an average particle size of less than about 0.4 microns that contains no particles larger than about 1.5 microns. Furthermore, in order to make high quality MLCC's from dielectric formulations based on BT, it is preferable that the BT powder be highly crystalline (i.e., it exhibits defined tetragonality), dense, stoichiometric, and pure. Furthermore, it is desirable that the BT powder also possesses a narrow and monomodal particle size distribution (hereinafter sometimes abbreviated as “PSD”). A BT powder possessing all of these properties could be referred to as an “ideal” material for making MLCC's.
Many processes have been developed over the years in an attempt to address the need for a fine particle “ideal” BT powder. The four most popular commercialized processes for making BT powder are known in the art as: (1) the solid-state process; (2) the oxalate process; (3) the hydrothermal process; and (4) the sol gel process. An excellent review of some of these processes is described by A. D. Hilton and R. Frost in an article entitled “Recent Developments in the Manufacture of Barium Titanate Powders”, Key Engineering Materials, Vols. 66 & 67, 1992, pp. 145–184.
BT powder (which is also sometimes referred to herein as “particles”) can be formed via a solid-state reaction between barium carbonate (BaCO3) and titanium dioxide (TiO2) at temperatures greater than 900° C. In general, BT particles obtained via the solid-state route are typically highly crystalline and dense. However, BT particles formed in this manner are subject to Ba—Ti based phase impurities, particularly when the starting mixture of barium and titanium reactants is not uniform or the reaction does not proceed to completion at the atomic scale. See, e.g., A. D. Hilton & R. Frost, “Recent Developments in the Manufacture of Barium Titanate Powders”, Key Engineering Materials, Vols. 66 & 67, 1992, pp. 145–184. In addition, BT particles formed by the solid-state route are often comprised of coarse aggregates with average particle sizes greater than 1 micron. See, e.g., D. F. K. Hennings, B. S. Schreinemacher, and H. Schreinemacher, “Solid-State Preparation of BaTiO3-Based Dielectrics, Using Ultrafine Raw Materials”, J. Am. Ceram. Soc., 84 [12], 2777–2782 (2001). The particle size distribution of BT particles formed by the solid-state route is usually not narrow and monomodal because of the high calcination temperatures necessary to complete the solid-state diffusion reaction. Thus, the coarse aggregates must be physically reduced in size using a de-agglomeration step (i.e., by pulverization and/or milling) in order to be useful in the manufacture of electronic components and devices. This de-agglomeration step increases the risk of introducing undesirable impurities into the material and, because intense energy is required to reduce the particle size, it can alter certain desirable properties of the material, such as crystallinity, which makes it less suitable for use in electronic components.
In view of the foregoing disadvantages, many alternative processing routes have been developed for the production of high-purity BT powder. Such alternative processing routes include microemulsion processing, coprecipitation processing, sol-gel processing, hydrothermal synthesis, molten salt reactions, processing from polymeric precursors, and various oxalate and citrate processing routes. A number of these prior art methods can be used to produce fine submicron BT powders of near uniform size, but they have not been widely practiced on a commercial scale. This is because they often require exotic manufacturing schemes, have low product yields, and/or use exotic and/or expensive precursor materials.
Some wet chemical processes such as sol gel and hydrothermal processes are practiced commercially. These processes can be used to produce BT powder that has a fine particle size and monomodal PSD, but the individual particles are known to possess a significant degree of internal porosity. See, e.g.: D. F. K. Hennings, C. Metzmacher, and B. S. Schreinemacher, “Defect Chemistry and Microstructure of Hydrothermal Barium Titanate”, J. Am. Ceram. Soc., 84 [1], 179–182 (2001); and J. Fukazawa, I. Osada, T. Shioya, K. Ochiai, S. Tanabe, and T. Kunieda, “Properties of Barium Titanate Powders for Thin Layer MLCCs Produced by Chemical Processes”, CARTS 2000: 20th Capacitor and Resistor Technology Symposium, 6–10 March 2000. This internal porosity is an undesirable feature that typically results in a BT powder that exhibits less tetragonality than BT powders derived from solid-state or oxalate routes. Moreover, when such powders are formulated and fabricated into MLCC's, they commonly yield lower dielectric constants and higher dissipation factors than MLCC's formed using BT powders derived from solid-state or oxalate routes. Of course these powders can be calcined at temperatures high enough to remove the inherent internal porosity, which increases the degree of tetragonality and density, however, this then compromises the fine particle size and often results in a large particle size fraction that is undesirable. BT powders made by these processes also tend to have higher costs of manufacture because of higher raw material costs and more complicated processing requirements.
A process for preparing BT powder described by Clabaugh, W. S., Swiggard, E. M., and Gilchrist, R., in an article entitled, “Preparation of Barium Titanyl Oxalate Tetrahydrate for Conversion to Barium Titanate of High Purity,” Journal of Research of the National Bureau of Standards, Vol. 56, No. 5, 1956, pp. 289–291, which is known in the art as the “Clabaugh Process”, is used conventionally to produce BT powder on a commercial scale. In the Clabaugh Process, a mixture of an aqueous solution of titanium oxychloride (TiOCl2), hydrochloric acid (HCl) and barium chloride (BaCl2) is added slowly to an aqueous solution of oxalic acid (C2H2O4) in a vessel at a temperature of about 80° C. and vigorously stirred to precipitate barium titanyl oxalate tetrahydrate (BaTiO(C2O4)2.4H2O) (barium titanyl oxalate is hereinafter sometimes abbreviated as “BTO”). The BTO is then washed with distilled water and vacuum filtered. After drying in air, the BTO is calcined at a temperature of about 900° C., or higher, to convert it to BT particles. After calcination, the BT particles are typically highly crystalline and dense, however the PSD is usually broad and bimodal rather than narrow and monomodal.
The Clabaugh Process, while widely used commercially, suffers from several limitations. As noted above, for example, it does not yield a BT powder having a fine particle size and a narrow and monomodal PSD. De-agglomeration of the large calcined aggregates requires a high-energy milling step that tends to significantly degrade the inherent tetragonality present in the pre-milled BT powder. The Clabaugh Process also requires the use of at least three reaction vessels with heating capability to run the reaction and typically results in product yields of only about 93 to 97%. The volumetric efficiency of the Clabaugh Process is also relatively low, generating only approximately 0.3 kgs of BT per gallon of reaction slurry.
As previously noted, the consumer demand for electronic product miniaturization requires thinner fired dielectric layers in MLCC's. In order to achieve this and maintain the integrity of the thinner MLCC's, the dielectric powders used to form these layers must also become finer in size. These new requirements have led to many variations of the Clabaugh process and in particular, the last decade has seen a strong renewed interest in the oxalate synthesis technology. Some examples of the research that has occurred in the field of BTO synthesis using a modification of the Clabaugh process are now described.
Frank Schrey studied the effects of pH on the chemical precipitation of barium-strontium titanyl oxalates and found that the level of strontium that can be doped into the BTO is a strong function of the pH used during the precipitation. See Frank Schrey, “Effect of pH on the Chemical Preparation of Barium-Strontium Titanate”, J. Am. Ceram. Soc., 48 [8], 401–405 (1965).
Yamamura et al. prepared BTO by adding an ethanol solution to the oxalic acid solution. See H. Yamamura, A. Watanabe, S. Shirasaki, Y. Moriyoshi, and M. Tanada, “Preparation of Barium Titanate by Oxalate Method in Ethanol Solution”, Ceramics International, Vol. 11 [1], 17–22 (1985). The authors of the article also studied the effects of starting reagents, reaction temperature, and titration rates. Under certain conditions they were able to produce very fine particles of crystalline barium titanate, having an average PSD of about 0.3 microns. However, it appears that the process is very dilute because of the large amounts of ethanol used. Thus the process would probably not be cost effective from a commercial perspective because of the large amounts of ethanol used in the reaction and for the washing of the product.
Wilson et al., U.S. Pat. No. 5,783,165, disclose a process whereby barium carbonate powder is added directly to an aqueous solution of oxalic acid and titanium oxychloride that is maintained at a temperature of between 30° C. and 90° C. The patent states that BTO made this way was calcined at 1125° C. for 5 hours and then jet-milled to de-agglomerate the powder. The resulting barium titanate was said to be fine grained, between 0.2 and 0.45 microns in average particle size. X-ray diffraction scans showed that the barium titanate was crystalline, but that it did not exhibit a high degree of tetragonal splitting as can be seen in their figures.
Hennings et al., U.S. Pat. No. 5,009,876, disclose a process in which the order of addition of reactants typically used in the Clabaugh Process is modified, and the reaction temperatures typically used in the Clabaugh Process are varied to obtain a finer particle barium titanate particle ranging between 0.2 and 0.5 microns. However, the patent discloses that the particles have a Ba/Ti ratio between 0.975 and 0.985, which is significantly less than the desired target of 1.000. No X-ray diffraction patterns are shown or described in the patent, so the degree of tetragonality for the milled BT powder is unknown.
BTO has been prepared from barium salts other than barium chloride. Examples are barium nitrate and barium acetate. See, e.g., W. E. Rhine, K. Saegusa, R. B. Hallock, and M. J. Cima, “Control of Ceramic Powder Composition by Precipitation Techniques”, Ceramic Transactions (1990), 12 (Ceramic Powder Science III), 107–118, wherein the authors studied the effects of reaction time and pH on BTO made from barium acetate or barium nitrate and titanyl ammonium oxalate.
Hari S. Potdar et al. precipitated BTO from extremely dilute solutions of barium acetate (0.01M) and potassium titanyl oxalate (0.01M). See H. S. Potdar, P. Singh, S. B. Deshpande, P. D. Godbole, and S. K. Date, “Low-Temperature Synthesis of Ultrafine Barium Titanate (BaTiO3) Using Organometallic Barium and Titanium Precursors”, Materials Letters, Vol 10 [3], 112–117 (1990). Upon decomposition of this powder at 550° C. for 6 hours they were able to produce crystalline BT particles that were <200 nm in size and also monodispersed and spherical. However, the X-ray diffraction pattern showed no evidence of any tetragonality and they did not measure Ba/Ti ratios for their powders.
In H. S. Potdar, S. B. Deshpande, and S. K. Date, “Alternative Route for Synthesis of Barium Titanyl Oxalate: Molecular Precursor for Microcrystalline Barium Titanate Powders”, J. Am. Ceram. Soc., 79 [10], 2795–2797 (1996), the authors describe a route for precipitating BTO by reacting an alcoholic solution containing butyl titanate monomer and oxalic acid dihydrate with an aqueous solution of barium acetate. By decomposing the BTO in air at about 1050° C. for 4 hours they generated a crystalline BT powder that does display some tetragonality, as can be seen in the X-ray diffraction pattern. However, no measurements of particle size or Ba/Ti ratio are given. The process is very dilute having a volumetric efficiency of less than 0.1 kg of BT per gallon of reaction slurry and a low product yield of only about 85%.
When comparing and contrasting all the existing commercial methods for making BT powder, it is apparent that there is presently no single practical method that can simultaneously produce a BT powder possessing all of the physical and chemical attributes that are required for a fine particle “ideal” BT powder. Therefore, a new chemical process is needed to address the deficiencies in the existing processes.